Cellular communication networks are designed for full-duplex operation, in which terminals (such as cellular telephones) can both receive and transmit signals simultaneously. Cellular network standards thus assume terminals have full-duplex capability and permit simultaneous downlink and uplink transmissions to and from any given terminal. In other words, terminals must generally be capable of receiving and decoding a downlink signal from a base station in the course of making an uplink transmission.
Cellular push-to-talk (PTT) is a new class of mobile telephone service that enables instant one-to-one and one-to-many half-duplex communications on cellular telephones, emulating “walkie talkie” operation. In half-duplex operation, a terminal can either receive or transmit, but cannot both receive and transmit at the same time. An open PTT-over-cellular (PoC) standard is emerging under the auspices of the Open Mobile Alliance (OMA). Generally, PoC is implemented as an add-on feature of full-duplex cellular telephones and requires that participating telephones listen continuously for signals on a channel that is allocated for PTT service.